1. Field of the Invention
This invention relates to methods of fabricating implantable medical devices such as stents.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial implantable medical device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of these endoprostheses. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
Stents have been made of many materials including metals and polymers. Polymer materials include both nonbioerodable and bioerodable plastic materials. The cylindrical structure of stents is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or planar films of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric carrier can include an active agent or drug. Furthermore, the pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. Longitudinal flexibility facilitates delivery of the stent and rigidity is needed to hold open a body lumen. The pattern should be designed to maintain the longitudinal flexibility and rigidity required of the stent. A stent should also have adequate strength in the circumferential direction.
A number of techniques have been suggested for the fabrication of stents from tubes and planar films or sheets. One such technique involves laser cutting or etching a pattern onto a material. Laser cutting may be performed on a planar film of a material which is then rolled into a tube. Alternatively, a desired pattern may be etched directly onto a tube. Other techniques involve cutting a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter.
A treatment involving a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region requiring treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding. stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.
It is desirable for a stent to have certain mechanical properties to facilitate delivery and deployment of a stent. For example, longitudinal flexibility is important for successful delivery of the stent. In addition, circumferential strength and rigidity and are vital characteristics in deployment and for holding open a body lumen. As indicated above, the pattern of the stent may be designed to provide longitudinal flexibility and rigidity.
However, the characteristics of the material of which a stent is composed also affects the mechanical properties of the stent. An advantage of stents fabricated from polymers is that they tend to possess greater flexibility than metal stents. Other potential shortcomings of metal stents include adverse reactions from the body, nonbioerodability, and non-optimal drug-delivery. However, a potential shortcoming of polymer stents compared to metal stents, is that polymer stents typically have less circumferential strength and rigidity. Inadequete circumferential strength potentially contributes to relatively high recoil of polymer stents after implantation into vessels. Furthermore, another potential problem with polymer stents is that struts can crack during crimping, especially for brittle polymers. Therefore, methods of manufacturing polymer stents that improve circumferential strength and rigidity are desirable. The embodiments of the present invention address the issue of improving circumferential strength and rigidity in polymer stents.